The invention relates to an apparatus for securing bones, and in particular, to a bone plate.
Bone plates are being used today in countless different embodiments and offer a form of assistance in osteosynthesis that has become indispensable, in particular in treating fractures where the two bone fragments on the proximal and distal sides of a fracture must become fused again. The bone plate serves to hold the bone fragments in the desired position in relation to one another so that the bone can become fused again as desired.
A special example of such a bone plate is known from WO-A-01/19268. In individual embodiments of the bone plate described there (see FIG. 2 and FIG. 3 in WO-A-01/19268), the hole axis of individual holes in the plate runs obliquely in relation to the underside of the plate while the upper face of the plate and the underside of the plate run parallel to one another. The inside wall of the plate hole has a peripheral burr which becomes deformed when the bone screw is screwed into the hole so that the screw becomes “blocked” in the plate hole, i.e., is secured in its tightened position.
The bone plates described in WO-A-01/19268 are disclosed specifically for tubular bones such as the tibia. Such bone plates have a great material thickness, so normally it does not present any problem to countersink the head of the screw in the screw hole even when the screw is tightened so that the actual direction of tightening deviates from the “ideal direction of tightening,” i.e., the direction of the hole axis, which is possible within certain limits. In addition, in the case of tubular bones such as the tibia, the operating field is usually readily accessible for the surgeon.
It proves to be more difficult to countersink the screw head in “thin” bone plates having a much smaller plate thickness, namely in particular when the actual direction of tightening the bone screw deviates from the “ideal direction of tightening” because then due to the smaller plate thickness the axial length of the hole is not very great. However, such comparatively “thin” bone plates are the rule due to the small amount of soft tissue available and also for aesthetic reasons, especially in the maxillofacial area.
WO-A-95/16403 discloses a bone plate in which the axes of the holes in the plate run at an angle different from 90° in relation to the plane of the underside of the plate. An elevation is provided on the upper face of the plate around each plate hole. To fasten the bone plate, a pilot hole is first created with a suitable drilling tool, which is guided into the oblique plate hole, so that the pilot hole runs precisely in the direction of the hole axis. In the subsequent tightening of the bone screw, the bone screw is guided in the pilot hole and is tightened in the ideal direction of tightening, namely in the direction of the hole axis. The cylindrical screw head can therefore be accommodated completely in the elevation in the bone plate. A type of blocking of the bone screw in the position in which it is completely screwed in is proposed here in such a way that recesses are provided in the outside wall of the cylindrical screw head and after the screws have been screwed in completely, the elevation in the bone plate is completely deformed into these recesses with the help of a suitable tool. However, the bone screw can be screwed in only exactly in the direction of the hole axis in the case of the bone plate known from WO-A-95/16403 because otherwise the cylindrical screw head cannot be accommodated in the elevation. Therefore, a pilot hole must also be created in the bone, guiding the desired direction of screwing in the screw precisely. However, this is complex and tedious for the surgeon (creating the pilot hole, etc.) and also in practical terms does not allow any deviation in the direction of tightening from the ideal direction of tightening.
However, the surgeon is frequently confronted with the problem of being unable to easily recognize the “ideal tightening direction” in the case of hole axes running obliquely and thus an oblique “ideal tightening direction” —especially when the surgical field is cramped. If the actual tightening direction of the bone screw from the ideal tightening direction deviates to a certain extent, the screw head could not be countersunk in the plate hole when using the bone plate described above. However, predrilling a pilot hole in the ideal direction of tightening is complicated and expensive and is especially problematical in terms of accessibility in the maxillofacial area. However, without a pilot hole, there is the problem for the surgeon of being able to recognize the “ideal tightening direction” easily, in particular when interoral access (access only through the mouth) is to be performed, but this can also occur in a case of access with a skin incision. The location of the skin incision cannot be selected at random due to the distribution of nerves in the maxillofacial area, so the screwdriver can then be guided at a right angle through the incision and will then encounter the bone plate. This would be a simple variant in terms of handling but it is often impossible to implement because of the distribution of the nerves. The same difficulty in selecting the location also occurs with a so-called “transbuccal” access in which a small hole is made through the skin (but not a large incision) and then the screwdriver is passed through this hole, for example. However, the plate itself may be introduced interorally and held at the desired location.